Geology of the Malheur Butte Quadrmtgk, MdkrCotlaty, by Ian P. Madin and Mark L. Ferns, Oregon Department of Geology and Mineral Industries,

Oregon Department of Geology and Mineral Tndustries Open-File Report 0-97-2 GEOLOGY OF THE MALHEUR BUTTE QUADRANGLE, MALHEUR COUNTY, OREGON

MTRODWCTLON

The Malheur Butte quadtangle is located in northern Malheur County, near the west marsin of the western plain. OIdest rocks exposed in the quadrangle are fine-grained silt- and sandstone deposited ktween 5 and 2 Ma dong the shores of a large I&, horn as Lake Tdallo (Cope, 1883; knks and Bonnichsen, 1989) that extended from eastern Oregon to central Idaho during the Pliocene. Fluvial gravels and tephra record Motmation and volcanism in a small, strike-slip basin neat the end of the

Pliocene. Younger units inciude three distinct terraces formed during subsequent downcutting by the

Snake and Malheur rivers. The upper two terraces constitute an important local sand and gravel resource.

Youngest units include s1ackwate.r deposits of the catastrophic Bonnevilie Fld(circa 14,000 B.P.), which underlie the fertile agricultural lands of northern Malheur county.

ACKNOWLEDGEMENTS

The geology of the Malheur Butte quadrangIe was mapped in I995 in cooperation with the Oregon

Department of Corrections, in order to determine whether seismogenic faults were exposed in the quadrangle. We would like to thank Kurt Othberg, Idaho Geological Survev, for his constructive review of the map and text EXPLANATION

Qal Allu~ialdeposits (Eolocene) Overbank fldand channel deposits of sand and silt along the

Malheur River. Mainly silt and fine sand derived from reworked loess and Bonneville Flood deposits

(Qsbfj. Locally includes fine- to medium-gained, volcanic-~lastgravels. Unit is overlain by prlp chained soils of the Qrrincy and Powder series (bvell, 1980).

Qaa Alluvial deposits Wolocene and upper Pleistocene) Pale gray to tan, thin-Mdad silt and fine- to medium-pined sand deposited on the alluvial plain adjacent to the Malheur River channel Depositional surface has been largely modified by modem agricultural oprations. Unit is 3 to 7 m thick and overlies the lowermost Snake River terrace (Qbg), Unit is overlain by prly drained soils of the Urnapine and

Stankid series (Lovell, 1980).

Qshf Bonne~ilteHood Deposits (upper Pleistocene) Massive to thinly Wed tan silt and fine sand deposited during the Bonneville Mood (Malde, 1968). Unit consists of 3 - 12 rn of overbank silt and fine

sand deposited on a broad knch adjacent to the Snake and Malheur rivers. Unit overlies a buricd terrace unit (Qbg) that appears only in water well logs. Outcrop corresponds to unit QtaE 1 of Gannett ( 1990).

Areal distribution is defined by the low bench at 2 180 A elevation south of the Malheur River. The low bchn marked by the well-drained mils of the %bee and Greenleaf sen= (Lmell, 3980). Age of the

Bonnmdle Flood is about 14,000 years B.P. (Scott and others, 1983; Malde, 199 I). PLEISTOCENE TERRACE DEPOSITS Three distinct terrace surfaces form a stepped series of gently indined surfaces in the quadrangle The terraces record successive stages of lateral planation of Idaho

Group lacustrine rocks by the ancestral Snake River. Pbw surfaces are covered with 3 - 17 m gravei, silt and sand Coarse cobble gravels at the 'base of each terrace were deposited in broad flood-plains cut into the older Idaho Group lacustrine rocks. Higher taram gravels are compriM largely of light colored granitic and porphyritic rhyolite clasts derived from the Idaho Batholith and Challis Volcanics respectively, both units exposed in Idaho to the east.

Qhg herterrace deposits (middle or upper Pleistocene) Fluvial gravel and sand deposits 3 to 117 m that are encountered only in water wells drilled through units Qas and Qsbf. Unit is not exposed at the surface and is shown only in cross section. Base of the unit, based on water wells, is at abut 2 130 ft elevation, about 20 ft lower than the present day Malheur river channel.. Unit is tentatively carrelat& w~ththe Whitney terrace of Burnham and Wood ( 1992) on basis of elevation and degree of incisement by the Snakc River to the east. Othberg and others (1995) suggest a minimum age of 107 ka for the Whitney terrace, based on a radiometric date on a basalt flow erupted during ;terraceincisement. Unit is correlative with unit Qtal 1 of Gaanett (1 990).

Qdg Intermediate terrace deposits (lower Pleistocene) Terrace deposit of silt, sand, and gravel 8 to 17 m thick Unit consists of a lower 3 - 8 m thick bad of gravel overlain by 5 to 10 m of water and wind lain silt and sand. Uppr fine-grained section typically consists of a pale gray, thinly laminated tuffamus siltstone overlain by a massive tuffaceous sandstone. Lower section typically consists of a matrix- supported, nell-mrted gravel with clams as large as 25 cm.Clasts include coarsely crystalline granitic and pegmatitic cDbMes Qpical of the Idaho Batholith and potassium feldspar-phjnc rhyolite porphyq cobbles typical of the Chalhs Volcarucs. Gravel matrix censists largely of coarse quartz, feldspar, and mica grains. Terrace surface is an eastward sloping tableland which drops from 2600 ft elevation on the wai to 2300 ft elevation on the east. Small streams have cut channels as deep as 80 ft through the terrace into underlying

Tig dments. Terrace surface in places is heavily mantled by loess and Bonneville Flddeposits (Qbsf)

Tentatively correlated on hisof degree of dissection and elevation, with the 1.58 Ma Deer Hat-Amity terrace of Othberg and others (1 995).

QTtg Upper terrace deposits (lower Pleistocene or upper Pliocene) Extensively eroded terrace 5 to 10 m thck of sand and mbble gravel. Cobble clasts are similar to Qdg clasts and include coarsely crystalline granitic and pegmatite cobbles and potassium feldspar-phyric porphyry rhyolite cobbles. Terrace forms isolated plateaus between 2700 and 2800 ft elevation on the west side of the quadrangle. Tentatively correlated on the basts of elevation and degree of dissection with the Tenmile Gravel terrace of Othkg and others (19951, who place the age of the Tenmile Gravel ktween 1.58 and 1.7 Ma.

IDAHO GROUP The sequence of lacustrine and fluviatile sediments and mafic lava flows deposited in the western Snake River plaln between the late Mimene and early Pleistocene comprise the Idaho Group

(Malde and Powers, 1962). In the quadrangle, units include:

Tig Glenns Ferv Formation (Pliocene) Fine- to mdum-grained sandstone and siltstone. h map area, unit consists of floodplain and fluvial facies. Fl-lain facies consists of light tan, thin-kdded tufiaceous silt- and claystone with lenses of fine-medium-grainedarkosic sandstone. Overlying flmlal units near Malheur Butte include massive and cross-bedded channel sands and finegrained pebble conglomerate. Upper age limit of the Glenns Feny Formation is cited as between 1.76 Ma (Othkrg and others, 1995) and 2 Ma W&,199 1). Oldest reported date in the Glenns Few Formation is a 3.23 + 0.4

Ma fission track date reported by Kimmel(1982) on ash near the hse of the formation The sequence of coarse conglomerate and hydrovolcanic deposits comprising the upper part ofthe Clem Feny Formation north of MaIheur Butte is mapped separately asm Tigg Conglomerate (Pliacene) Interbedded deposits of weakly lith~fiedcoarse conglomerate,

sandstone, and vitric lapilli tuff. Unit fills a narrow channel inc~sedInto the uppr part of the

Glenns Ferry Formation north of Malheur Butte. Clasts include well-rounded coarse grained

granite and pegmatite, and rounded black andesitc and basalt. obsidian, and silicified tuff

cobbles. Dark gray lapilli tuff crops out as 2 - 3 ft thick ledges in the upper part of the unit. Large

(r 2 fi diameter) blocks of aphyic lava locally occur with the gravel cobbIes. Pliocene age based

on tentative correlation of the lapilli tuff and aphyric lava blocks with the Malheur Butte volcano,

which has been radiometrically dated at 2.78 & 0.83 Ma (L&s, 1994).

Tmv Malheur Butte volcanics mictcene) Vent oomplex of bluish-gray, platy aph!?ic lava flows, pyroclastic deposits, and subvolcanic intrusion. Unit includes the er&d core of a small volcano erupted through and into the Glenns Fmy Formation. The vent plug is comprised of complexly intermix4 andesite - with sparse plagioclase phenocryms- and glassy aphyric black kite(Lees, l994). Pliocene age based on Ara0/A29age of 2.78 10.84 Ma (Lees, 1994).

GEOLOGIC HISTORY

The Glenns FeqFormation, the oldesr unit exposed in the quadrangle, rmrds Pliocene sedimentation

along the fluvial fldpla~ddeltaicmrgn &Lake Idaho, a large lake that oocupisd most of the western

Snake River plain dmng the late Miocene and Pliocene (Cope, 1883; Jenks and Bonnichsen, 1989). Thin- bedded, he-grained hlffaceous Glenns Ferry Formation silt- and claystones exposed at Malheur Butte are

l~thologiessuggstive of a floodplain setting. Overly~ngcross-Wed sandstone and pebble conglomerates

are indicative of a fIwiaI or deltaic environment. Regi~nallydenswe upwmlaarsening of the Glenns

Rny Forma~ionhas been noted elsewhere by Othberg (1 994) who suggests that the lake margin shrank

during a period of climate change in the Northern Hemisphere. Upward coarsening in the Glenns Ferry Fomtion near Malheur Butte can also 'be attributed to faulting along the nwgin ofthe westerlk Snake River p1aiu. The Mallmr Butte fault zone is a N15W trending zone of faults and folds approximately 7000 ft in width that extends northward from Malheur Butte The fault zone is a north- northwest- trending rift or rhom~hasmpartially filled with syntectonic channel gravels and tephra SWce-slip component of fadung is suggested by folding in lower Glenns Ferry

Formatioll silt- and claysfones. Withilk the N15"W trending-zone, a fold axis and parallel s~~lall displacelnent faults trend NO% - NIOOW. Channel gravels (mapped seprtrateIy as unit Tigg] exposed in the top of the GEenns Ferq Formation have been cut by small intrapben faults. Gray lapilIi tuffs presumably erupted from Malheur Butte to the south are intmkdded with the gravels and provide evidence for syndqmsit~onalvolcanism. Northward transport n suggested by the presence of large aphyric lava blocks almost certainly derived from Malheur Butte to the south. Age of faulting is somewhat constrained by the Malheur Butte volcano, situated on the south end of the fault zone. Silt-, clay-, and sandstone in the lower Glenns Ferry Formation were folded and hydrothennally aitered porto the at

2.78 f 0.84 Ma mption of Malheuf Butte. mset of beds in the ripper Glenns Ferq Formation gravels indicate some post-volcanic faulting

During the late Pliocene, the ancestral Snake River carved a terrace across the Glenns Ferry Formation, leaving a I 5 - 30 ft thick layer of cobble gravel (unit Ttg). The change from pdonlinantly lacustrine and fluvial-deltaic shoreline deposits of the Glenns Feny Fornation to the high energv fluviatile terrace grave1 deposits mrdthe capture of the ancestral Snake River by the system and the emptying of Lake Idaho through Hells Canyon (Wheeler and Cook, 1954). Initial stage of downcutting formed the

Tenmile Gravel and Tuana Gml(Maldc, 199 1) an4 tentatively, the Ttg terrace. These gravels are the oldest remnants of the high-energy drainage system developed following the Snake River capture. The older terrace had been extensively dissected by about 1.58 Ma, when a younger terrace grave1 was deposited across the Ontario Heights. A temporary retam to a lamstsine setting is indicated by the widespread deposition of tuffaceaus sib- and fine-grained sand on top of basal Qdg gravels lncisement by the Snake River resumed with formation of a third terrace -tentatively correlated with the Whitney

Terrace- in the late Pleistocene.

The Mdheur River had partially cut through the lower terrace when, at about 14,000 years B P. most of the quadrangle was buried by 20 - 30 ft of silt and fine sand deposited by the Snake River during the late

Pleistocene BannevilIe flood (Malde, 1968; Gannett, 19N). Downstram hydraulic damming at Fam~ll

Bend formed a floodad backwater whose high stand reached an elevation of at least 2417 fI (Othberg and others, 1995). Wind-blown silt (loess) and sand picked up from the flooded lowlands subsequently mantled much of the adjoining highlands Bonnmlle Flood depos~tsalong the Malheur River floodplain have been reworked by periodic flooding by the Malheur River into modem alIuvim

SRUCTURF, AND SEISMIC HAZARDS

A major fault zone cuts Pliocene units in the southwest comer of the quadrangle. The Malheur Butte fault zone, shown by Brown and others (1 980) extends northwst of Malheur Butte for several tens of miles.

The Mall~eurButte fault zone is a N1 Sowtrending zone of faults aud folds approximately 7000 ft in width. The faul zone is clearly visible on air photographs as a parallel series of major Iineaments that cross Glenns Ferry Formation units kaltilting of lower Glenns Perry Formation strata is readily apparent along small displacement faults (<3 ft) exposed in the banks of the *bee canal. Smke-dip component of faulting is suggested by folding of lower Glems Ferry Formation silt- and claystones.

Within the NlSW trendu~g-zone,fold axis and parallel small displacement faults trend NO% - N1O"E.

The Malheur Butte fault zone is tentatively interpreted as a north- northwest- trending rift or rhornb- chasm that was partially filled with syntectonic channel gavels and tephra during a prid of strike-slip faulting and volcanism.

A similarly oriented series of small, normal displacement faults are exposed to the west at Vale (Brown,

1982). These make up part of the Vale fault zone, considered by Lawrence (1976) to he the notthemmost of a series of l&-lateral strike-slip faults that terminate the northern margin of the .

Undeforrned terrace gravels atop the fault trace indicate that Malheur Butte fault zone has probably not scen acllve movement since the nuddle Pliocene and is not currently active.

GEOLOGIC RESOURCES

Sand and gravel, used for aggregate in the local construction industy, are the main mineral resources mined in the quadtangle. Active mines situated in the upper (QTtg) and intermediate (Qdg) terrace5 Past mines have generally stand where the lower gravel unit in the intermdate Qdg terrace has been expwed by channels cut through the terrace. Economic feasibiliv of past operations on the intermediate terrace appear to have been dependent upon working shallow gravels Mining of the intermediate Qdg terrace usually ceased when the gravel is mined back to where the mmplete 5 - 10 m sectton of overlying sand and silt is prwerved. Although there is less cover on the upper terrace gravels, the quantity of gravel is limited. Thick gravels in the Tigg unit have thick overburden and are of lirmted areal extent.

A hydrothermal afteration zone of about 80 acres adextent is exposed on the east flank of Malheur

Butte. Most of the zone consists of bedded silt- and claystone that have been altered m situ to a greenish- brow.clay. Although the site is listed as a possible bentonite clay resource (Gray, 1993) the clay does not appear to have simcant swelling characteristics.

Narrow ribs of silicified sandstone that crop out northeast of Malheur Butte presumably mark weakly altered fault zones. The most intensely altered zone 1s ex@ on the east side of Malhew Butte, where weakly silided breccias crop out adjacent to the unalteted volcanic neck of Malheut Butte. Although the potential for precious metal resources at this site appear sl~ght,sigmficant hot spring-type mineralization occurs along similarly orient4 fault zones at Vale Buttes, to the east, and Tub Mountain, to the northwest. Potentla1 energy resources include natural gas and geothermal. Although natural gas INS been identified in water wells drilled into the Idaho group, exploration to date has failed to dei5ne a natural gas resource in the western Snake River plain (Newton and Corcoran, 1963) Similarities between the Malheur Butte fault zone arrd the faults in tl~eKnown Geothermal Resource Area at Vale (Brown and others, 1980) may afford some slight potential for geothermal resources in the quadrangle. REFERENCES

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